Well logging pad member constructed to contorm to borehole wall curvature



SEARCH mun Jaw-am- TO BOREHOLE WALL CURVATURE Filed April 2, 1965 5 Sheets-Sheet l Apnl 23, 1968 D. F. SAURENMAN 3,379,963

WELL LOGGING PAD MEMBER CONSTRUCTED TO CONFORM TO BOREHOLE WALL CURVATURE Filed April 2, 1965 5 Sheets-Sheet 2 April 23, 196% D. F. SAURENMAN 3,379,963

WELL LOGGING PAD MEMBER CONSTRUCTED T0 CONFORM TO BOREHOLE WALL CURVATURE Filed April 2, 1965 5 Sheets-Sheet United States Patent 3,379,963 WELL LQGGENG PAD MEMBER CON- STRUCTED TO CONFGRM T0 BORE- WALL (IURVATURE Dean P. Ssnrenman, Friendswood, Tex., assignor, by

mesne assignments, to Schlumberger Technology Corporation, Houston, Tern, a cerporation of Texas Filed Apr. 2, 1965, Ser. No. 445,087 13 Claims. (Cl. 324--) ABSTRACT OF THE DISCLSSURE This disclosure shows a borehole wall-engaging pad member having electrodes which can be utilized to investigate earth formations traversed by a borehole. The pad member has a central relatively stiff portion and two adjacent pad portions which are constructed to flex or bend with respect to the central portion. To accomplish this, a metal sheet, which may be perforated is embedded in an elastomeric material which is of a conductive nature on the borehole wall-engaging face of the adjacent pad portions. The metal sheet provides rigidity and springiness to the adjacent pad portions as well as providing an equal potential distribution to the conductive elastomeric material.

This inven ion relates to electrical apparatus for investigating subsurface earth formations traversed by a borehole and, more particularly, to apparatus for measuring the electrical resistance properties of a subsurface earth formation by means of electrodes in the borehole adapted to be pressed against the borehole wall.

One method of investigating subsurface earth formations traversed by a borehole is to move a system of pad mounted wall-engaging electrodes through the borehole and to determine the resistance presented by the earth formations to the flow of electrical current emitted from one or more of the electrodes. The electrical record or log obtained from this manner of investigation aids in determining the nature and lithological character of the various subsurface earth formations. The resulting data is useful in the case of oil well boreholes in that, among other things, it enables the presence and depth of any oil or gas bearing strata to be determined.

In drilling a borehole by the usual rotary method, the borehole is filled with a drilling mud during the drilling process. This drilling mud will penetrate radially into the subsurface earth formations for a distance dependent upon the porosity of the earth formations. When the drilling mud invades a permeable strata, there is a mudcake remaining on the wall of the borehole. This mudcake is caused by the solid particles of drilling mud being unable to pass through the permeable strata. The mud filtrate which actually invades the permeable zone alters the electrical resistance properties of the earth formation immediately adjacent to the borehole wall and, not uncommonly, increases the resistance of such portions, particularly where fresh muds are used. However, the resistivity of the mud cake which is formed on the borehole wall is usually relatively low compared to the resistivity of the flushed zone.

For the case of pad-mounted electrodes which engage the wall of the borehole, as the mudcake thickness increases, more and more of the electrode current is short circuited back to the borehole current return point by the relative low resistance path formed by the mudcake. This makes the measurements of the flushed zone resistivity and thus, any subsequent formation porosity determination more difficult, because the measurement is influenced to a large degree by the mudcake resistivity. This problem is overcome to a large extent by using a 3,379,963 Patented Apr. 23, 1968 wall-engaging electrode system of the focused type. Systems of this type are to be found in US. Patent No. 2,712,629, granted to H. G. Doll on July 5, 1955, and in US. Patent No. 3,132,298, granted to H. G. Doll and I. L. Dumanoir on May 5, 1964. In a wall-engaging electrode system of the focused type, a current How is emitted by one of the electrodes and used for determining or surveying the formation resistance characteristics. This surveying current flow is constrained to a desired lateral flow pattern by a focusing current emitted by another one of the electrodes which is adjacent to the main surveying electrode. Focusing current emitted by means of the focusing current electrodes opposes any tendency of the survey current to flow in an undesired direction, as for example, along the mudcake. Thus, most of the survey current is caused to penetrate laterally into the earth formations for some appreciable distance. As indicated by the Doll and Dumanoir patent, more elaborate precautions are required as the environment becomes more severe or the requirements on the measurements become more demanding.

The electrode system described in the Doll and Dumanoir patent provides an improved measurement of the resistivity of the invaded zone region of the earth formations even for a case of relatively thick mudcake. Among other things, this is accomplished by utilizing relatively large electrode surfaces on the wall-engaging pad member. It has been found, however, that in some boreholes, undesired variations appear in the measurements. After considerable effort, it has been found that the undesired variations in these instances are caused primarily by relatively large changes or variations of the shape or contour of the borehole wall. If the crosssectional shape of the borehole remains fairly circular and the diameter fairly constant, results are satisfactory. If, however, a section of the borehole assumes an elliptical or other non-circular shape and if, at the same time, a fairly thick mudcake is present on the borehole wall, then the measured value will be somewhat different depending upon what side of the ellipse is being engaged by the electrode system.

It has been found that this undesired effect can be largely eliminated if a constant relationship can be maintained between the curvature of the borehole wall and the curvature of the wall-engaging face of the electrodes. This requires providing different electrode system curvatures for the different sides of the ellipse, depending upon which side of the ellipse the electrode system is pressed against.

The relationship between the formation resistivity and the measurement made with an electrode system is described by the formula:

R=KITV efficient in the above formula. Therefore, changes in the electrode system curvature will tend to introduce corresponding changes into the measured values. Since these changes are independent of the formation resistivity, they are undesirable.

It is an object of the present invention therefore, to

provide new and improved apparatus for measuring electrical resistance properties of subsurface earth formations adjacent to a borehole.

It is another object of the present invention to provide new and improved wall-contact electrode apparatus of the focused type which provides improved focusing action under adverse borehole conditions.

It is a further object of the present invention to provide a new and improved focused-type wall-contact electrode system which is less effected by variations in radius of curvature of the borehole wall.

It is an additional object of the invention to provide a new and improved focused-type wall-contact electrode system which can adapt itself to changes in the curvature of the borehole wall without introducing excessive changes in the calibration constant of the system.

In accordance with the present invention, apparatus for investigating earth formations traversed by a borehole comprises a pad member having a forward face adapted to be pressed against a bolehole wall. This pad member includes a first pad portion having at least one electrode forming part of said forward face and a pad portion adjacent to the central pad portion formed of at least one electrode and an elastomeric material, said electrode including conductive supporting means in the elastomeric material, said supporting means being arranged to enable the adjacent pad portion to bend so that the pad member may more nearly conform to the curvature of the borehole wall.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.

Referring to the drawings:

FIG. 1 illustrates, partially in cross-section, a representative embodiment of borehole investigating apparatus constructed in accordance with the present invention;

FIG. 2 is an enlarged front view of the wall-engaging face of one of the electrode pads of the FIG. 1 apparatus;

FIG. 3 is a cross-sectional view of the electrode pad of FIG. 2 taken along the section line 3--3 thereof;

FIG. 4 is a crosssectional view of the electrode pad of FIG. 2 taken along the section line 44 thereof;

FIG. 5 is a cross-sectional view of the electrode pad of FIG. 3 taken along the section line 55 thereof, the electrode pad being disposed in a borehole with current flow lines indicated therein, and an electrical schematic of the circuitry of the present invention; and

FIGS. 6 and 7 are cross-sectional views corresponding to views taken along the section line 6, 7-6, 7 of FIG. 2 but showing modified forms of construction for the present invention.

Referring to FIG. 1 of the drawings, there is shown a representative embodiment of a downhole portion of an apparatus constructed in accordance with the present invention for investigating a formation 10 traversed by a borehole 11, the borehole 11 being filled with a conductive fluid or drilling mud 12. This apparatus includes an elongated support member 13 adapted for movement through the borehole 11. The support member 13 includes an upper instrument housing portion 14 of a generally cylindrical shape having a hollow fluid-tight interior for enclosing certain downhole electrical circuits to be discussed hereinafter. The support member 13 also includes intermediate frame or chassis portion 15 in the form of an iron or steel I-beam or equivalent structure and a lower nose portion 16 of a generally cylindrical shape.

Supported from an intermediate I-beam portion 15 of the support member 13 on opposite sides thereof are a pair of electrode pad members 17 and 18, each of which is adapted to be urged against the wall of the borehole 11. The pad member 17 is supported by way of support arms 19 and 20, which are pivotally coupled to the pad member 17 and the I-beam portion 15. Similarly, pad member 18 is supported by way of support arms 21 and 22, which are pivoted to both the pad member 18 and I-beam portion 15. A suitable actuating mechanism for extending and retracting support arms 19 and 21 is included Within the lower portion of the instrument housing section 14.

The downhole portion of the apparatus also includes a current return electrode B located on the support member 13 close enough to the location of the pad member 17 so as to be electrically proximate thereto, but longitudinally spaced apart from the location of the pad member 17 so that no portion of this B electrode is located directly behind the pad member 17. This B current return electrode is of generally cylindrical shape and is mounted on suitable electrical insulation material which either covers or forms the nose portion 16 of support member 13. Electrical connection to the B current return electrode is made by way of an insulated conductor 26 which extends upwardly along the l-beam portion 15 to the instrument housing 14.

The downhole support member 13 is suspended in the borehole 11 by way of an armored multi-conductor cable 27 which extends upwardly to a suitable drum and winch mechanism (not shown), located at the surface of the earth for raising and lowering the support member 13. The first hundred feet or so of the cable 27 immediately above the instrument housing portion 14 is covered with a layer of electrical insulation material 33 such as rubber. Supported on this layer of insulation material 33 towards the upper end thereof are an electrically remote current return electrode B and an electrically remote potential reference electrode N.

Only the electrode pad member 17 will be dealt with in the explanation of this invention. The pad member 18 can be any downhole testing apparatus, the particular apparatus not being a feature of this invention.

Considering the electrode pad member 17, the FIG. 2 "iew of electrode pad member 17 shows the face of the pad that is pressed against the borehole wall. An electrode, by definition, must be made of a material which is electrically conductive, and must never be made of insulation or non-conductive material. The major portion of pad member 17 is formed by a generally rectangular pad portion A The wall-engaging portion 24 of the A electrode is made of an electrically conductive material and, as such, constitutes a focusing current electrode for emitting focusing current from the various surface portions thereof. The A electrode has a recess 23 formed in a central portion of the wall-engaging face thereof, which recess 23 is of a rectangular shape. The recess 23 is filled with a suitable electrical insulation material 25 such as rubber.

The electrode pad member 17 also includes an elongated electrode A of rectangular shape centrally located in recess 23 and electrically insulated from the A electrode proper by the portion 25a of insulation material 25 contained within the recess 23. The surface portion of the A electrode is thus centrally located relative to the Wall-engaging portion 24 of the A electrode. The A electrode has a length D in a direction parallel to the borehole axis which is greater than one-half the length of the electrode A and a width D greater than one-quarter the lenght D Electrode A constitutes a survey current electrode for emitting survey current into the adjacent earth formations.

The electrode pad member 17 further includes a potential monitor electrode M located intermediate the survey current electrode A and the focusing current electrode A and electrically insulated from the survey current electrode A by the portion 25b of insulation 25 and from the focusing current electrode A by the portion 25a of insulation 25. Insulation portion 25'!) is also a suitable insulation material such as rubber. This potential monitor electrode M has an exposed surface portion of narrow width which defines a path encircling the A survey current electrode.

Referring again to FIG. 1, the electrical connections to electrodes A M, and A are made by way of insulated conductors 28, 29 and respectively. These insulated conductors 28, 29 and 30 pass upwardly through the hollow interior of the support arm 19 to the electrical circuits contained within the instrument housing portion 14, or if support arm 19 is not hollow, as for example, a channel shaped member, then conductors 28, 29 and 30 are secured to support arm 19 to pass upwardly to instrument housing portion 14. Mechanical connections of the electrode pad member 17 to the support arms 19 and 20 is made by way of lug members 31 and 32 respectively. The electrical continuity of the lug members 31 and 32 may be broken by means of non-conductive inserts.

Referring now to FIG. 3, there is shown a cross-sectional view of electrode pad member 17 taken along the section line 3-3 of FIG. 2. The section line 33 is sectioned in FIG. 2 so that the main :parts of the cross-section of electrode pad member 17 can be shown in one figure. As a result, the section line 3-3 is not straight and the electrodes A M, and A are somewhat more removed from the front portion of pad member 17 than if section line 33 were straight. The dotted line and clear, non-hatched portions of electrodes A M, and A show the electrode portions, which portions are within the area where the cross section line 3-3 of FIG. 2 varies from the middle of pad member 17, as if section line 3-3 were straight down the middle of pad member 17 of FIG. 2. The survey current electrode A of pad member 17 is centrally located with respect to the other electrodes of pad member 17 and is embedded in a rubber insulation material 25. The survey current electrode A is relatively stiff and of a suitable conductive material, such as iron. Adjacent to survey current electrode A on both sides is the monitor electrode M, made of a suitable conductive material which is relatively stiff, such as iron. On the upper and lower part of the electrode portion of electrode pad member 17 and adjacent to monitor electrode M is focusing current electrode A Focusing current electrode A comprises a flexible electrically conductive reinforcing member 36 which is embedded in an electrically conductive elastomeric material 35. The electrically conductive elastomeric material is preferably a relatively resilient material, such as conductive synthetic rubber, which for example, can be made by embedding small metal or carbon particles in rubber during the manufacturing process. On the back side (or non-wall-engaging side) of reinforcing member 36 and fixed thereto is an insulation or non-conductive elastomeric material 34. The elastomeric materials 34 and-35 will bend when pressure is applied to one end thereof, but will regain their previous configuration when the pressure is removed. Embedded in the conductive elastomeric material 35 is reinforcing member 36, which is preferably made from a relatively hard non-flexible electrically conductive material such as iron, but which is constructed in such a manner as to be flexible. Thus, reinforcing member 36 can be constructed of perforated sheet metal or a wire screen or cloth. Reinforcing member 36 should have spring-like qualities so that after bending or deforming, it will tend to return to its former position and shape. Thus, it can be seen that focusing current electrode A can be bent, but will return to its original position and shape when the bending pressure is released.

Reinforcing member 36 is preferably placed close to the wall-engaging surface of this portion of focusing electrode A but is, nevertheless, still covered by conductive rubber 35 so that there is a layer of conductive rubber between the metal reinforcing member 36 and the borehole wall. This is to insure that the metal reinforcing member 36 will not become caught on uneven points along the borehole, which could damage the electrode pad member 17. Besides providing reinfoncing strength to focusing electrode A metal reinforcing member 36 pro vides a highly conductive path to all portions of focusing electrode A thus providing uniformity of potential throughout. This is desirable since there is usually some resistance associated with conductive rubber. There is, therefore, only a short path through which the focusing current must travel through conductive rubber 35 to the wall-engaging portion 24 thereof, which short path has a negligible effect, if any, on disturbing the potential uniformity along the face 24 of focusing electrode A Electrodes A M, and A are insulated from one another by insulating rubber 25. Insulating rubber 25 extends from focusing current electrode A on the upper part of the electrode portion of pad member 17 to electrode A on the lower part of the electrode portion of pad member 17. Adjacent to insulating rubber 25 is a metal plate 37 which extends between the upper and lower parts of focusing electrode A Metal plate 37 is embedded in a portion of the insulation rubber 34 adjacent to the upper and lower parts of focusing electrode A Metal plate 37 is connected with reinforcing member 36 by a metal member 39 on the lower portion of focusing electrode A and metal member 38 on the upper portion of focusing electrode A Located adjacent to metal plate 37 and secured thereto is another metal plate 40. Adjacent to metal plate 40 are two insulating blocks 42 made of plastic material, only one of which is visible in FIG. 3. Metal plate 40 is bent along its edges, which edges are embedded in plastic blocks 42. FIG. 4 shows both plastic blocks 42. The edge of metal plate 40 that is embedded in plastic block 42 is designated 41 in FIG. 3. Located adjacent to plastic blocks 42 is another plastic block 43, which extends from the top portion of pad member 17 to a point near the bottom of pad member 17. Adjacent to plastic block 43 is a metal plate 44 to which is attached metal lug members 31 and 32. Support arms 19 and 20 (shown in FIG. 1) are connected to lug members 31 and 32. Plastic blocks 42 and 43 and metal plate 44 are secured together by several screw 45 and nut 46 pairs, shown at different locations in FIG. 3. Plastic insert 47 is inserted in the vacancy left by screw 45 at each screw location.

Referring now to FIG. 4, there is shown a cross-sectional view of electrode pad member 17 taken along the section line 4--4 of FIG. 2. 'It can be seen in FIG. 4 that the ends 41 of metal plate 40 are embedded in plastic locks 42 to secure the portion of pad member 17 facing the borehole wall with the portion attached to support member 13. Plastic block 43 is located between plastic blocks 42 on either side thereof and extended between metal plate 40 and metal plate 44 in a T shape. Plastic block 34 also extends between plastic blocks 42 and metal plate 44. A plurality of screws 45 and nuts 46 are shown securing metal plate 44, plastic block 43, and plastic blocks 42 together. Thus, all parts of electrode pad member 17 are securely fastened together and electrodes A M and A are insulated from one another and from the other metal pieces associated with electrode pad 17 by the plastic and rubber insulation pieces.

Concerning the electrical connection to electrodes A M and A of electrode pad member 17, there is shown in FIG. 3 a metal connecting piece 48 embedded in insulating rubber 25 and connected to monitor electrode M. Inserted in metal connecting piece 48 is a circular electrical connector 51, the head portion 49 of which is embedded in insulating rubber 25, to which is connected the electrical conductor 29 that connects the circuit elements in housing 14 with monitor electrode M.

Looking now at FIG. 5, there is shown a cross-sectional view of electrode pad 17 taken along section line 5--5 of FIG. 3. Metal connector 48 is connected to monitor electrode M by a dotted line (the actual mechanical connection cannot be seen in FIG. 5, because of the location of section line 55., but is represented by a dotted line to gain a better understanding of the electrical schematic of FIG. 5), metal connectors 49 are connected to survey current electrode A and a metal connector 50 is connected to metal plate 37, which is connected to the metal reinforcing member 36 of focusing current electrode A There is shown a conductor 52 connecting the A electrode connectors 50 together, although separate conductors could connect each connector 59 separately to the circuit contained within housing 14.

Turning now to the schematic diagram portion of FIG. 5, the circuit portion contained within the dash line box 14 corresponds to the circuit portions that are located Within the interior of the instrument housing portion 14 of FIG. 1. The electrical connections to the electrodes on pad member 17 are made at the pin 51 in each connector. The circuits are constructed to maintain the potential level V of the monitor electrode M relatively constant with respect to a remote reference point and, at the same time, to determine the formation resistance characteristics by measuring the resulting variations in the magnitude of the survey current I emitted from the central A electrode. Alternating-current reference voltage V is supplied from source 81 by way of input transformer 82a, a high-gain amplifier 82, and a conductor 28 to the A focusing current electrode. Also supplied to the input transformer 82a of the amplifier 32 via conductor 29 is a signal representative of the potential, V of the potential monitor electrode M. This potential signal V is supplied with like polarity to the side of the transformer 82a primary winding opposite from the reference voltage V so that if the monitor electrode M is a potential equal to the reference V then the net input signal to the amplifier 82 is nearly zero. If, on the other hand, the potential level V of the monitor electrode M differs from the V, value, then the amplifier 82 input signal, which is in the nature of a degenerative feedback error signal, serves to adjust the amplifier output current I which is supplied to the A electrode so as to bring the monitor voltage V back to the desired V, value. Thus, the potential level of the monitor electrode M is kept substantially constant with respect to the remote reference point represented by electrode N.

In addition, the potential difference between monitor electrode M and the A survey current electrode is ad justed so as to be maintained at a value of substantially zero. To this end, the monitor electrode signal V is also supplied to an input transformer 83a of a high-gain amplifier 83. A signal representative of the potential level of the A electrode is supplied to the opposite side of the primary winding of the input transformer 83a. This A signal is supplied by way of conductor 85. Thus, if the M-A potential difference is not substantially zero, then the error signal existing at the input of amplifier 83 serves to adjust the amplifier output current I which is supplied to the A electrode by way of conductor 30 to establish this desired zero potential difference.

As the formation resistivity in front of the electrode pad 17 varies, different amounts of survey current I will be required to maintain the M-A potential difference at a zero value. The magnitude of this survey current required will be directly proportional to the electrical conductivity of the formation material. This indication is transmitted to the surface of the earth by means of transformer 86 having a low impedance primary winding coupled in series between amplifier 83 and conductor 30 leading to the A electrode. Thus, the voltage signal appearing across the secondary winding of transformer 86 is proportional to the I current flow and therefore to the formation conductivity. This voltage signal is transmitted to a galvanometer unit 87 at the surface of the earth by way of cable conductors 88 and 89.

Instead of using the constant voltage circuit of FIG. 4, a constant current circuit such as shown in US. Patent No. 3,132,298, supra, could be used. As a further alternative, both the voltage and current may be varied and a suitable ratio circuit or device used for taking a ratio of voltage to current or vice versa for providing the desired output signal.

Electrode pad member 17 is shown pressed against a borehole wall 11 lined with a mudcake 53 in FIG. 5. The dotted line 17a indicates the free space shape of electrode pad 17, that is the shape of electrode pad 17 if there were no borehole wall. The solid line 17 indicates the shape of electrode pad 17 in the borehole when the radius of curvature of the borehole is less than the radius of curvature of the surface of electrode pad 17. Thus, it can be seen that the novel design of electrode pad 17 enables the electrode surface portion of pad 17 to conform to variations in the borehole diameter. However, while the focusing electrode A changes shape substantially in conformance with the shape of the borehole, it can be seen that the solid non-flexible portion of pad member 17, electrodes A and M, do not change shape and thus are disposed at a small distance from the mudcake 53. It has been found, however, that it is not so critical to have the survey and monitor electrodes, A and M, in contact with the mudcake 53 as it is to have focusing electrode A in contact with the mudcake. When sur vey electrode A is not in contact with the mudcake, the potential set up by focusing electrode A will not allow the survey current emitted from survey electrode A to diverge. On the other hand, when focusing electrode A is at a distance from mudcake 53, the focusing current emitted therefrom will diverge rapidly back to the mudcake since there is no potential to keep it from doing so.

Now concerning the focusing action of the present invention, there are seen current flow lines I and I in FIG. 5. Survey current electrode A emits survey current I into the earth formation surrounding the borehole, which current diverges back to the current return electrode B (see FIG. 1). The divergence of the survey current I back to the current return electrode B becomes more severe when the mudcake 53 lining the borehole wall 11 is thick, in which case some of the survey current I will flow through the mudcake 53 to the current return electrode B However, the focusing current I emitted from focusing current electrode A directs the survey current i deeper into the earth formation by setting up a potential gradient concentrically surrounding survey current I which maintains survey current I in a longitudinal flow away from the borehole.

However, when the mudcake 53 is very thick, and the borehole radius of curvature becomes non-uniform around the circumference of the borehole, an electrode pad of non-flexible construction could not conform to the radius of curvature of the borehole wall at all circumferential points around the borehole. If that electrode pad of nonfiexible construction were placed on that portion of the borehole where the radius of curvature of the borehole is greater than the radius of curvature of the electrode pad and the mudcake thickness fairly substantial, some of the focusing current I emitted from focusing electrode A wouldtend to diverge more quickly back through the mudcake 53 to the mud-filled portion of the borehole proper. Thus, the survey current l emitted from survey current electrode A would also tend to diverge too quick- 1y.

However, with the flexible features of the present electrode pad member 17, the focusing current electrode A will be pressed against the mudcake 53 lining the borehole wall on any circumferential portion of the borehole wall even when the radius of curvature of the borehole wall differs around its circumference. As seen by the current flow lines of FIG. 5, the focusing current I emitted from focusing current electrode A extends laterally outward into the earth because the focusing current electrode A is pressed tightly against the mudcake 53 lining the borehole wall. Thus, there is very little divergence of the focusing current I; emitted from points on the focusing current electrode A, which are nearest to the survey current electrode A which, in turn, extends the survey current I emitted from survey current electrode A laterally outward for an appreciable distance. Thus, it can be seen that there is very little divergence of the survey current I under adverse borehole conditions and the accuracy of the resistivity or conductivity readings will be improved under such conditions.

When the physical configuration of an electrode pad member is varied, the proportionality constant K of the device will vary. However, with the novel form of construction of electrode pad 17 shown in the present invention, there will be very little variation in K. One reason for the small variation in K is that the electrodes on the front side of electrode pad 17 are electrically insulated from the back side of electrode pad 17 by the non-conductive rubber 23 and 34 and plastic blocks 42 and 43. Thus, because the front and back sides of electrode pad 17 are insulated from one another, a single value of K can be utilized in determining the resistivity or conductivity of the adjoining earth over a Wide range of radius of curvatures of the borehole.

Referring now to FIG. 6, there is shown another embodiment of the present invention. Both FIG. 6 and 7 are cross-sectional along a line corresponding to section lines 6, 7-6, 7 of FIG. 2, but with certain modifications to the FIG. 2 embodiment, as set forth hereinafter. In FIG. 6, a group of bare metal wires 54 have been attached to the metal reinforcing member 36 in an axial direction with the borehole along the focusing electrode A which metal wires 54 are embedded in the conductive elastomeric ma terial 35. Metal wires 54 would provide better conductivity between metal reinforcing member 36 and the surface portion 24 of focusing current electrode A which portion is contacting the mudcake surrounding the borehole, without affecting the desirable flexibility characteristics of the electrode pad member 17 shown in FIGS. 25. The top and bottom ends of metal wires 54 would bend over away from the surface portion 24 and would be embedded in insulation rubber 34 so as to prevent contact of any jagged ends of wires 54 with the borehole wall. Thus, only the rounded portions of wires 54 would be exposed to the borehole wall, preventing them from being caught on uneven parts of the borehole Wall, and at the same time, allowing substantially perfect conductivity between metal reinforcing member 36 and the borehole wal-l.

Referring now to FIG. 7, there is shown a further embodiment of the present invention. In this embodiment, electrodes A and M have metal reinforcing members embedded in conductive rubber in the same fashion as focusing electrode A Survey current electrode A has a flexible metal reinforcing member 55 embedded in a conductive rubber 56. In the same fashion, monitor electrode M has a flexible metal reinforcing member 57 embedded in a conductive rubber 58. By this construction of electrode pad member 17, electrodes A and M would have the same flexibility as focusing electrode A Thus, it can be seen that the entire electrode pad will have a maximum degree of flexibility, allowing all surface portions of the electrode pad to be in contact with the mudcake on the borehole wall over a wide range of radius of curvatures of the borehole wall.

While there have been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

one electrode of said array being comprised of an electrically conductive material of nondiexible construction forming part of the forward face of said pad member; and

another electrode of said array forming another part of said forward face and comprised of a sheet-type metal material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to more nearly conform to the curvature of the borehole wall said other electrode including an electrically conductive elastomeric material on the forward face of said other electrode.

2. Apparatus for investigating earth formations traversed by a borehole comprising:

a support member;

a pad member adapted to be pressed against a borehole wall, said pad member having an electrode array on the side facing the borehole wall and a metal body, for attaching the pad member to the support member, on the side farthest from the borehole wall;

an electrically non-conductive body separating said electrode array from said metal body;

a first electrode of said array being comprised of an electrically conductive material of non-flexible construction; and

a second electrode of said array comprised of a sheettype metal material of flexible spring-like construction embedded in an elastomeric material, the front wall-engaging portion of said second electrode being comprised of a conductive elastomeric material, and an electrically non-conductive elastomeric material fixed to the back side of said second electrode.

3. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a first electrode of said array being comprised of an electrically conductive material of non-flexible construction and forming a portion of said forward face; and

a second electrode of said array forming another portion of said forward face and comprised of a flexible, spring-like, sheet-type metal body embedded in an elastomeric material so that the second electrode will bend upon the borehole radius of curvature becoming less than the electrode pad radius of curvature, the front Wall-engaging portion of the second electrode being comprised of a conductive elastomeric material covering the sheet-type metal body, the sheet-type metal body providing a substantially equal potential distribution on the borehole wallengaging face of the second electrode.

4. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a first electrode of said array being comprised of an electrically conductive material of non-flexible construction and forming a portion of said forward face; and

a second electrode of said array forming another portion of said forward face and comprised of a sheettype metal material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to bend about a relatively fixed longitudinal axis between the first and second electrodes to more nearly conform to the curvature of the borehole wall, an electrically conductive elastomeric material comprising the forward wall-engaging face portion of said second electrode, and an electrically non-conductive elastomeric material fixed to the back, non-wall engaging side of said second electrode.

5. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a first electrode of said array being comprised of an electrically conductive material of non-flexible construction forming a portion of said forward face;

a second electrode of said array forming another portion of said forward face and defining a concentric path around the first electrode and insulated therefrom; and

said second electrode comprised of a flexible, springlike metal body extending concentrically outward from said first electrode, said metal body embedded in an elastomeric material so that the second electrode will bend upon the borehole radius of curvature becoming less than the pad member radius of curvature, the borehole wall-engaging forward face of the second electrode being comprised of an electrically conductive elastomeric material.

6. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a first electrode of said array being comprised of an electrically conductive material of non-flexible construction and forming a portion of said forward face;

a second electrode of said array comprised of a metal material of non-flexible construction forming another portion of said forward face and defining a concentric path around the first electrode and insulated therefrom;

a third electrode of said array forming another portion of said forward face and defining a concentric path around the first and second electrodes and insulated therefrom; and

said third electrode comprised of a flexible, springlike metal body extending concentrically outward over a substantial distance from said first and second electrodes, said metal body embedded in an elastomeric material so that the third electrode will bend upon the borehole radius of curvature becoming less than the pad member radius of curvature, the borehole wall-engaging forward face of the third electrode being comprised of an electrically conductive elastomeric material.

7. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a first electrode of said array forming a portion of said forward face and being comprised of electrically conductive material of non-flexible construction;

circuit means for energizing the first electrode for emitting survey current into the adjacent earth formation;

.1 second electrode of said array forming another portion of said forward face and comprised of a nonfiexible type material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to more nearly conform to the curvature of the borehole wall, the borehole wallengaging portion of the second electrode being comprised exclusively of an electrically conductive elastomeric materials; and

circuit means for energizing the second electrode for emitting focusing current into the adjacent earth formation.

8. Apparatus for investigation earth formations traversed by a borehole comprising:

a support member; a pad member adapted to be pressed against a borehole wall, said pad member having an electrode array on the side facing the borehole wall and a metal body, for attaching the pad member to the support member, on the side farthest from the borehole wall;

an electrically non-conductive body separating said electrode array from said metal body;

a first electrode of said array being comprised of a solid metal material of non-flexible construction; circuit means for energizing the first electrode for emitting survey current into the adjacent earth formation;

a second electrode of said array comprised of a sheettype metal material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to more nearly conform to the curvature of the borehole wall, the front wall-engaging portion of said second electrode being comprised of an electrically conductive elastomeric material, and an electrically non-conductive elastomeric material fixed to the back side of said second electrode; and

circuit means for energizing the second electrode for emitting focusing current into the adjacent earth formation.

9. Apparatus for investigating earth formations traversed by a borehol comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array comprising at least two electrodes; and

at least two electrodes of said array forming portions of said forward face and comprised of a metal material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to more nearly conform to the curvature of the borehole wall, the borehole wall-engaging forward face of the electrode being comprised exclusively of an electrically conductive elastomeric material.

10. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

one electrode of said array being comprised of a solid non-flexible material and forming part of said for ward face;

another electrode of said array forming another part of said forward face and comprised of a non-flexible type material of flexible spring-like construction embedded in an elastomeric material for enabling the pad member to more nearly conform to the curvature of the borehole wall, the borehole wall-engaging forward face of said electrode being comprised of an electrically conductive elastomeric material; and

a plurality of metal conductors embedded in said electrically conductive elastomeric material and connected to said non-flexible material of flexible springlike construction on the side thereof facing the borehole wall.

11. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a non-flexible longitudinally extending first portion of the forward face of said pad member forming at least one electrode; and

a longitudinal extending second portion of the pad member, adjacent to the first portion, formed of at least one electrode and an elastomeric material, said electrode forming part of said forward face and including conductive supporting means in the clastomeric material, said supporting means being arranged to enable the adjacent pad portion to bend about a relatively fixed longitudinal axis located be- 13 tween the central and adjacent pad portions so that the pad member may more nearly conform to the curvature of the borehole wall.

12. Apparatus for investigating traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wal-l, said pad member member having an electrode array;

a non-flexible longitudinally extending first portion of the pad member having at least one electrode forming part of said forward face; and

a longitudinally extending second portion of the pad member, adjacent to the first portion, formed of at least one electrode and an elastorneric material for enabling the adjacent pad portion to flex so that the pad member may more nearly conform to the curvature of the borehole wall, the electrode on the adjacent pad portion formed of a flexible, spring-like metal sheet embedded in an elastomeric material which is conductive on the borehole Wa11-engaging forward face of the adjacent pad portion.

13. Apparatus for investigating earth formations traversed by a borehole comprising:

a pad member having a forward face adapted to be pressed against a borehole wall, said pad member having an electrode array;

a non-flexible longitudinally extending central portion of the forward face of said pad member forming at least one electrode; and

earth formations 14 longitudinally extending portions of the pad member, adjacent to the central pad portion, formed of at least one electrode and having an elastomeric material for enabling the adjacent pad portion to bend about a relatively fixed longitudinal axis between the central and adjacent pad portions so as to more nearly conform to the curvature of the borehole wall, the electrode on the adjacent pad portion having a flexible, spring-like metal member embedded in an elastomeric material which is conductive on the borehole wall-engaging forward face of the adjacent pad portions.

References Cited UNITED STATES PATENTS 2,649,960 8/1953 Gammeter 324-54 X 2,696,589 12/1954 Bendix et a1. 324-54 2,712,629 7/1955 Doll 324-10 X 2,712,630 7/1955 D011 324-10 X 2,778,951 1/1957 Tittman 324-10 XR 2,961,600 11/1960 Tanguy 324-10 XR 3,068,403 12/1962 Robinson 324-54 X 3,069,620 12/1962 Servos 324-54- 3,093,793 6/1963 Hicken 324-54 3,132,298 5/1964 Doll et a1 324-10 RUDOLPH V. ROLI-NEC, Primary Examiner.

WALTER L. CARLSON, Examiner.

G. R. STRECKER, Assistant Examiner. 

